The present invention relates to a method for controlling a part- or full-time four-wheel drive vehicle and, more particularly, to a four-wheel drive vehicle controlling method in which distribution of the drive force to the front and rear wheels and change of the transmission gear ratios or positions are undergone by a hydraulic pressure control.
Regular small-sized vehicles have recently been equipped with four-wheel drive because of their advantageous running stability. The four-wheel drive can be classified into a part-time type, in which transmission of the drive force to the front or rear wheels can be interrupted at will, and a full-time type, in which the drive force is always distributed to both the front and rear wheels through a differential. In the prior art, these types of four-wheel drives are also combined with an automatic transmission and an auxiliary transmission which can shift the output of the automatic transmission between high and low speeds.
This combination has been proposed by an automatic transmission for the four-wheel drive with an auxiliary transmission, which is exemplified by Japanese Patent Application No. 7772/1985. This proposal is constructed of: a main transmission for changing the gear ratios by switching the manual valve with the shift lever and by switching the hydraulic pressure supply lines in response to signals such as the throttle openings or the number of revolutions of the output shaft; an auxiliary transmission for shifting the output of the main transmission between a high-speed drive, having a reduction gear ratio of 1, and a low-speed drive, having a reduction gear ratio larger than 1, by controlling applications and releases of the clutch and the brake through the hydraulic pressure control; and a transfer for shifting the two- and four-wheel drives by controlling application and release of the clutch through the hydraulic pressure control. In this automatic transmission, the valves of the hydraulic pressure control unit are switched depending upon the throttle openings and the number of revolutions of the output shaft to automatically change the gear ratios of the main transmission. If one of the drive modes including a high-speed two-wheel drive (H2), a high-speed four-wheel drive (H4) and a low-speed four-wheel drive (L4) is selected by the shift lever of the auxiliary transmission, the values in the hydraulic pressure control unit for the auxiliary transmission are switched to cause engagement or release of the clutch and the brake. This brings the auxiliary transmission into the high- or low-speed drive mode and the transfer into the two- or four-wheel drive mode.
Generally in the automatic transmission, the hydraulic pressure established by an oil pump is regulated to a line pressure by a primary regulator valve and to a throttle pressure, depending upon the throttle openings, by a throttle valve. These regulated hydraulic pressures are used to actuate various valves and hydraulic servos. As a result, the gear ratio changing controls are performed with proper timings or by proper operating forces. In the auxiliary transmission, also, the shifting controls are performed under predetermined hydraulic pressure. In the automatic transmission thus operated, however, a shifting (e.g., upshifting or downshifting) is automatically caused in the main transmission by the changes in the throttle openings and accordingly in the vehicle speed, whereas the drive modes are shifted between the two- and four-wheel drives independently of the shifting, by the manual selection of the shift lever. This may result in simultaneous occurrence of the gear ratio change at the main transmission and the change between the four- and two-wheel drives at the transfer. In case a shift is to be made from the high-speed two-wheel drive to the low-speed four-wheel drive, moreover, the auxiliary transmission is required to supply the hydraulic pressure not only to the clutches and brakes for the high- to low-speed drive shift but also to the clutches for the four-wheel drive. Thus, in the case of the simultaneous occurrence of the gear ratio change at the main transmission and the shift of the drive mode from the high-speed two-wheel drive to the low-speed four-wheel drive at the auxiliary transmission and the transfer or in the case of the simultaneous occurrence of the shift from the high-speed drive to the low-speed drive at the auxiliary transmission and the shift from the two-wheel drive to the four-wheel drive at the transfer, the number of the hydraulic elements to be supplied with the hydraulic pressure is temporarily increased. If, in this case, an accumulator is disposed in the supply passage of the hydraulic pressure, the hydraulic pressure supply must be notably increased. In either case, the hydraulic pressure will drop to a level lower than the designed value. As a result, the automatic transmission of the prior art has failed to set the engaging timings and/or forces of the clutches, brakes and so on, as desired. This may be accompanied by disturbances in the hydraulic characteristics of the transmission and accordingly encountered by difficulties in improvements in the gear ratio changing shocks or in fine adjustments of the torque distribution, thus raising the possibility of losing the smooth gear ratio changing characteristics and the expected merit of the four-wheel drive vehicle.